1. Field of the Invention
This invention relates to a method of increasing the strength of a porous body of carbon or graphite in which the pores of the body are filled at least partly with one or several carbonizable impregnating media and the impregnating media are carbonized in the pores of the body by a heat treatment.
2. Description of the Prior Art
In the manufacture of carbon and graphite members it is known to mix coke, graphite, carbon black and other carbon-containing substances of suitable grain fineness with a carbonizable binder such as tar pitches or resins, and to form bodies, by pressing, compacting or casting from such mixtures. The thus formed parts are heated in the absence of air to about 1000.degree. C. for carbonizing the binder and, if indicated, to about 2800.degree. C. for converting the carbon into graphite. Pores are produced in the bodies, especially in the carbonizing phase, by pyrolysis of the binders and the transport of gaseous dissociation products formed in the process within the bodies. The number and size of the pores are determined in large measure by the kind and the distribution of the starting materials, the respective shaping method and the condition of carbonizing.
There has been no lack of attempts and numerous methods have become known to reduce the porosity of carbon and graphite bpdoes by impregnation with a carbonizable substance, particularly to eliminate the adverse effect of the pores on the strength of the bodies and to decrease at the same time the permeability of the bodies for gases and liquids. Impregnating media are, for instance, bituminous coal tar pitches, petroleum pitches or hardenable resins such as phenol-formaldehyde resins or furan resins which are introduced alone or as a mixture, if necessary at elevated pressure, into the bores of the carbon or graphite bodies. The pitch-impregnated bodies are heated directly to a temperature of about 1000.degree. C. to convert the impregnating medium into carbon. For bodies impregnated with resin it is necessary to precede this with a heat treatment which is specific to the respective type of resin. The impregnating medium stored in the pores is pyrolyzed during the heat treatment, forming coke and gaseous decomposition products where again open pores, i.e., pores accessible from the surface, are produced in the bodies. For improving the strength of the bodies substantially and to reduce the permeability, it is therefore necessary to repeat the cycle consisting of impregnation, hardening if required, and carbonization of the impregnating medium several times. It is advantageous to use for each subsequent impregnating step an impregnating medium with lower viscosity as with a progressing number of cycles, the diameter of the pores and thereby, the transport velocity of the impregnating medium in the pores becomes smaller because of the embedment of the carbon. However, since impregnating media with a lower viscosity have in general also less coke residue, the effectiveness of the method decreases steadily with the number of repetitions, so that the desired improvement may require a large number of cycles.
According to another method, it is known to densify carbon and graphite bodies by precipitation of carbon from the gaseous phase inside the pores of the bodies and to improve the strength of the latter. The bodies heated to a temperature between about 800.degree. and 1600.degree. C. are exposed to hydrocarbon-containing gases such as propane, benzene vapor, etc. and a suitable carrier gas, which diffuse at least in part into the pores and are decomposed at the hot walls of the pores, precipitating carbon. The precipitation of the carbon within the pores requires accurate matching of the process conditions such as temperature, concentration and pressure difference, to the respective characteristic of the body, such as dimensions, porosity and distribution of pore size, and continuous changing of the conditions with reference to the instantaneous degree of change of size of the pores. Large-size bodies and bodies with complicated shapes, particularly, cannot be impregnated satisfactorily with carbon by this method. The properties of the bodies are not sufficiently improved, as the carbon is preferentially precipitated at the outer surface or at certain zones or portions of the body.